1. Field of Invention
The present invention pertains to the field of image sensors. More particularly, this invention relates to efficient dark current subtraction for an image sensor.
2. Art Background
A typical image sensor includes a set of sensing elements which generate electrical charge in response to light. For example, one type of image sensor is a charge-coupled device (CCD) array. A typical CCD array includes an array of CCD sensing elements and circuitry for sampling the charge levels of the CCD sensing elements.
A variety of images sensors including CCD arrays have light sensing elements which accumulate an electrical charge even in the absence of light. The electrical charge that accumulates in a sensing element in the absence of light is commonly referred to as dark current. Typically, the dark current in an image sensor creates an undesirable xe2x80x9cdarkxe2x80x9d image that overlays the optical image.
One prior method for removing the effects of dark current in an image sensor is to perform dark current subtraction. Typically, an image is obtained from the image sensor when it is illuminated by light from an image scene and an image is obtained from the image sensor when it is not illuminated. The image obtained when the image sensor is illuminated may be referred to as an image frame. The image obtained when the image sensor is not illuminated may be referred to as a dark frame. The dark frame is usually subtracted from the image frame to yield an image frame without the dark current component.
Prior methods for dark current subtraction usually obtain a dark frame each time an image frame is obtained because the dark current typically varies according to temperature and exposure. Unfortunately, the time consumed in obtaining a dark frame each time an image frame is obtained usually slows the rate at which useful images may be obtained with an image sensor.
A method for dark current subtraction is disclosed which enables a dark frame to be reused for dark current subtraction for multiple image frames. The dark frame is reused by scaling it according to changes in the dark current levels associated with the dark frame and the image frames. In one embodiment, the changes in dark current levels are determined by examining changes in charge samples from optically black sensing elements and dummy samples from circuitry in the image sensor.
Other features and advantages of the present invention will be apparent from the detailed description that follows.